Need Help: 2 digit(ie 15) input logic numpad

Hey i have to make a 4 bit calculator.
I have desgined most of it. i want to make a keypad(0 to 9) that will be able to give input up to 15. I have thought of one design. it would work like this :

i used 16 to 4 decimal to bcd encoder and a quad 2 bit multiplexer(74157).and a 4 bit adder.

the out put of encoder goes to data selector and adder which adds 10 to it.
if user press 0 then 4 the 4 comes out of the dataselector , if the user press 1 it changes the output of dataselector. when u again press the the digit the next output with 10 added (ie 12) goes out of the dataselector.

but i dont thing it is a good design. If any one can help me out by suggesting improvement to this design or by telling me some other design.

USe HEx keypad?

you are making your own right(maybe you can buy)

* and # ---- are -----E and F